Method and system for calibration of a voltage controlled oscillator (VCO)

ABSTRACT

A method for calibrating a voltage controlled oscillator (VCO) comprising applying a plurality of known voltages to the input of a VCO, monitoring, for each of the voltages, an output count from the VCO over a set interval, and storing the output counts for each voltage. Also disclosed is a system for calibrating a voltage controlled oscillator (VCO) comprising a plurality of known voltages, wherein the known voltage are connectable to the VCO, and a controller coupled to the output of the VCO, wherein the controller maintains a calibration table of VCO output counts for selected voltage inputs.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. patent application Ser. No.10/644,542, entitled “SYSTEM AND METHOD FOR MEASURING CURRENT;” U.S.patent application Ser. No. 10/644,625, entitled “A SYSTEM FOR ANDMETHOD OF CONTROLLING A VLSI ENVIRONMENT;” and U.S. Pat. No. 6.954,706,entitled “A METHOD FOR MEASURING INTEGRATED CIRCUIT PROCESSOR POWERDEMAND AND ASSOCIATED SYSTEM,” filed concurrently herewith, thedisclosures of which are hereby incorporated by reference herein intheir entirety.

BACKGROUND

In order to optimize the operating conditions of an integrated circuit,such as a microprocessor, it is often desirable to measure the currentused by the circuit. The standard method for measuring a current is tosend the current through a sense resistor of known value while measuringthe voltage drop across the resistor. From the known resistance and themeasured voltage, one may calculate the current load using Ohm's law(I=V/R).

It is important to get an accurate voltage measurement so that thecurrent can be precisely measured. One method of measuring voltage in adigital circuit uses a voltage controlled oscillator (VCO). When avoltage to be measured is applied to the control input of the VCO, theoutput of the VCO produces a series of pulses. A counter can be coupledto the VCO output and may count the VCO output pulses for a fixedinterval. The VCO output frequency can then be correlated to a knowninput voltage, thereby determining the value of the unknown inputvoltage. However, the accuracy of this method may vary because thevoltage versus frequency relationship for the VCO is not linear. Thegain of the VCO may vary with voltage, temperature, age and otherparameters. Therefore, the output count for a particular input voltagewill vary for different operating conditions.

SUMMARY

One embodiment of the invention includes a method for calibrating avoltage controlled oscillator (VCO) comprising providing a plurality ofknown voltages, applying the known voltages to the input of a VCO,monitoring, for each of the voltages, an output count from the VCO overa set interval, and storing the output counts for each voltage.

Another embodiment of the invention includes a system for calibrating avoltage controlled oscillator comprising a plurality of known voltages,wherein the known voltage are connectable to the VCO, and a controllercoupled to the output of the VCO, wherein the controller maintains acalibration table of VCO output counts for selected voltage inputs.

Another embodiment of the invention includes a computer program productcomprising a computer usable medium having computer readable programcode embedded therein, the computer readable program code comprisingcode for providing a plurality of known voltages, code for applying theknown voltages to each of the VCOs, code for monitoring an output countfrom the VCO over a set period, and code for storing, for each VCO, atable of output counts for each voltage.

Another embodiment of the invention is a system for calibrating avoltage controlled oscillator (VCO) comprising means for applying aplurality of known voltages to the input of a VCO, means for monitoring,for each of the voltages, an output count from the VCO over a setinterval, and means for storing the output counts for each voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating an embodiment of the presentinvention;

FIG. 2 is an exemplary calibration table according to embodiments of theinvention;

FIG. 3 is a flowchart illustrating a process for calibrating a VCO; and

FIG. 4 is a flowchart illustrating an alternative process forcalibrating a VCO.

DETAILED DESCRIPTION

Some embodiments of the invention generate tables that relate VCO inputvoltages to VCO output counts. The table may then be used to determineunknown input voltages using the output counts. Using an on-die, stablevoltage source and a set of matched, on-die resistors, a range of knownprecision voltages may be generated. The VCO control input can beconnected to these voltages. A unique output count is generated for eachinput. The VCO is allowed to run for a fixed time interval at eachvoltage input. A counter counts the VCO output frequency during thattime interval. At the end of each time interval, the values of the knowninput voltage and the output count are entered into a calibration table.The input voltage is swept across a range of input voltages and thecalibration table is completed for a range of voltages.

The VCO is susceptible to temperature and voltage variations, so thecalibration table may be updated continuously to reflect the currentoperating conditions of the device. The calibration table can also beused to determine the voltage drop across a known resistance as part ofa current measurement.

FIG. 1 is a block diagram illustrating an embodiment of the presentinvention. Components of a processor, such as CPU cores, monitoringcircuits and micro-controllers (not shown), may be constructed asintegrated circuits on VLSI CPU die 100. Components used in embodimentsof the invention are also constructed on the same VLSI CPU die 100. VCO101 is an integrated voltage controlled oscillator, which may beconstructed using any now known or later developed techniques. When avoltage is applied to control input 102, VCO 101 generates output pulsesat output 103. Counter 104 may be coupled to output 103 to count thepulses from VCO 101. As input voltage 102 varies, the output count atoutput 110 varies proportionally. For example, increases in voltage 102cause the frequency to increase at output 103 and the count to increaseat 110.

Controller 105 may be an on-die microprocessor that selects the inputvoltage for VCO 101 and that monitors the count value on counter 104.Controller 105 builds a calibration table for VCO 101 using the knownvoltage inputs.

Resistor ladder 106 may be constructed on VLSI CPU die 100. Resistorladder 106 may comprise 156 matched resistors connected in seriesbetween a known voltage and ground. The voltage drop across eachresistor is identical thereby creating a string of evenly spaced, knownvoltage taps V_(n) 109 between each pair of resistors R_(n) 107, R_(n+1)108. It will be understood that other voltage sources that producereliable known voltages may also be used.

One or more of the known voltages V_(n) are applied to input 102 on VCO101. Counter 104 monitors output 103 and measures the count 110 over aset time interval for each known voltage. Controller 105 builds a tableof known voltages mapped to output counts for VCO 101.

FIG. 2 is an exemplary calibration table 200. Voltage inputs 201 are theknown voltage values from resistance ladder 106. Output counts 202 arethe respective counts that are generated for each of the input voltages.Output counts 202 represent a number of pulses counted over apredetermined interval of time. Alternatively, output count 202 may be acount measured at output 110 for each control voltage 201. Althoughcalibration table 200 uses each voltage from ladder 106, in otherembodiments only a subset of these voltages are used for calibration.For example, only voltages from an anticipated operating range of inputvoltages for VCO 101 may be used to generate table 200.

Table 200 reflects the output count for known voltages under theoperating conditions for VCO 101 at the time the counts are made. Asconditions vary, such as a change in temperature, the output counts foreach input voltage may also vary. Therefore, controller 105 maycontinually update table 200 to reflect the current operating conditionsof VCO 101.

Once table 200 has been generated, an unknown voltage can be applied toinput 102. The VCO output is monitored for the unknown voltage andcounted for the same predetermined period of time. The count is thencompared to column 202 in calibration table 200 and is matched to theclosest calibration count. The value of the unknown input voltage isdetermined to be the respective voltage in column 201. For example, ifan unknown voltage generates a count equivalent to Count_(n) 203, thenthe unknown voltage is determined to be equivalent to voltage V_(n) 204.It will be understood that interpolation or other functions may be usedto determine the unknown input voltage when the output count fallsbetween two values in column 202.

FIG. 3 is a flowchart illustrating process 300 for calibrating a VCO. In301, a serial resistance ladder is provided with known voltage taps.These voltage taps may be evenly spaced voltages between a series ofresistors of the same value, for example. One of the known voltages isapplied to a VCO control input in 302. The input voltage causes the VCOto output a series of pulses. Those pulses are counted over an intervalof time in 303. The pulse count is added to a table in 304. The tablemaps known input voltages to measured output counts. In 305, the processis repeated for other voltages so that the calibration table can befilled for all available known voltages or for voltages in a selectedoperating range. Once the table is filled for all of the selectedvoltages, the process may be repeated again for each known voltage toupdate the count values to reflect current operating conditions.

In 306, an unknown voltage is applied to the VCO input. The output ofthe VCO may be monitored and, in 307, counted over a predeterminedinterval. The count is compared to the calibration table in 308 and iscorrelated to a known voltage.

FIG. 4 is a flowchart illustrating a process for calibrating a VCO. In401, a plurality of known voltages are applied to the input of a VCO. In402, the output count from each of the VCOs is monitored over a setinterval. In 403, the output counts for each voltage are stored.

1. A method for calibrating a voltage controlled oscillator (VCO)comprising: providing a serial resistance ladder having a plurality ofequal resistors; applying a plurality of known voltages to the input ofa VCO, wherein the known voltages are available from between each of theresistors; monitoring, for each of the voltages, an output count fromthe VCO over a set interval; and storing the output counts for eachvoltage.
 2. The method of claim 1 wherein the serial resistance ladderand the VCO are constructed on the same integrated circuit.
 3. Themethod of claim 1 further comprising: interleaving a VCO calibrationcycle in which known input voltages are substituted for measurements ofunknown input voltages.
 4. The method of claim 1 further comprising:repeating calibration operations for the same known input voltage atperiodic intervals to compensate for variations in operating conditions.5. The method of claim 1 further comprising: applying an unknown voltageto the VCO input; monitoring an unknown output count from the VCO over aset interval; and comparing the unknown output count to a table ofstored output counts.
 6. The method of claim 5 wherein the unknownvoltage is measured across a sense resistor with a known resistance, andfurther comprising: determining the value of the unknown voltage byrelating it to a known voltage that has an equivalent output count; andcalculating a current through the sense resistor using the knownresistance and a value determined for the unknown voltage.
 7. A systemfor calibrating a voltage controlled oscillator (VCO) comprising: aplurality of known voltages, wherein the known voltage are connectableto the VCO; and a controller coupled to the output of the VCO, whereinthe controller maintains a calibration table of VCO output counts,monitored over a set interval, for selected voltage inputs; and aresistance ladder having a plurality of voltage taps, wherein thevoltage taps provide the known voltages.
 8. The system of claim 7wherein the VCO, the resistance ladder and the controller areconstructed on the same integrated circuit.
 9. The system of claim 7further comprising: a plurality of VCOs; and wherein the controllermaintains a separate calibration table for each of the VCOs.
 10. Acomputer program product comprising a computer usable medium havingcomputer readable program code embedded therein, the computer readableprogram code comprising: code for selecting a voltage to be applied toinputs of a plurality of voltage controlled oscillators (VCOs), whereinthe voltage is selected from one of a plurality of voltage taps on aserial resistance ladder, wherein the voltage taps provide knownvoltages; code for monitoring output counts from each of the pluralityof VCOs over a set period of time, while the selected voltage is appliedto the VCOs' inputs; and code for storing, for each of the plurality ofVCOs, a table of output counts associated with the selected voltage. 11.The computer program product of claim 10 further comprising: code forinterleaving a VCO calibration cycle during which with other VCOmeasurements.
 12. A system for calibrating a voltage controlledoscillator (VCO) comprising: means for applying a plurality of knownvoltages to the input of a VCO; means for monitoring, for each of thevoltages, an output count from the VCO over a set interval; means forstoring the output counts for each voltage; means for applying anunknown voltage to the VCO input, wherein the unknown voltage ismeasured across a sense resistor with a known resistance; means formonitoring an unknown output count from the VCO over a set interval; andmeans for determining the value of the unknown voltage by relating it toa known voltage that has an equivalent output count; and means forcalculating a current through the sense resistor using the knownresistance and a value determined for the unknown voltage.
 13. Thesystem of claim 12 further comprising: means for interleaving a VCOcalibration cycle in which known input voltages are substituted formeasurements of unknown input voltages.
 14. The system of claim 12further comprising: means for repeating calibration operations for thesame known input voltage at periodic intervals to compensate forvariations in operating conditions.